1. Field of the Invention
The invention is a non-contact precision optical sensor for measuring distances to objects (targets) using coherent optical detection and two laser sources in a counter-chirp frequency modulated (FM) laser radar configuration
2. Description of the Related Art
Many optical systems exist which measure a distance to a target. Such systems utilize an open beam propagated through free space between the laser source and the target. However, when the target location is such that limited free space is available for beam propagation, such known systems are of limited use. Thus, known systems may be able to perform distance measurements, but the open beam optical sensor head prevents application in limited access areas and tight places. For example, precision measurement of dimensions inside a chassis cannot easily by accomplished with known open beam systems. While it is known to transfer light through optical fibers, precision is compromised due to the environmental effects on the fiber itself. These environmental effects change the optical path length and the polarization of the light in the fiber, adversely affecting measurement precision.
A known optical measurement system is disclosed in U.S. Pat. No. 4,340,304 to Massie. Massie discloses an interferometic method and system for detecting defects in the surface of a mirror. Massie discloses a polarizing beamsplitter, a quarter-wave plate, and a target (test mirror). However, Massie is an open beam system and thus incapable of accessing limited space targets.
The advantages of coherent optical detection are fundamental. The information carrying capacity of the optical beam reflected from the target is orders of magnitude greater than other available systems. Simply put, the use of optical heterodyne detection allows for optical radiation detection at the quantum noise level. As such, coherent optical systems provide greater range, accuracy, and reliability than many known prior art measurement systems. Coherent optical systems can also provide a greater scanning range, a greater working depth of field, and may also operate in ambient light conditions. Furthermore, in a coherent system the target beam is not required to dwell upon the target for very long in order to obtain sufficient information about the characteristics of that target location.
Briefly, optical heterodyne detection provides a source light beam which is directed to a target and reflected therefrom. The return light beam is then mixed with a local oscillator light beam on a photo detector to provide optical interference patterns which may be processed to provide detailed information about the target. Optical heterodyne techniques take advantage of the source and reflected light beam reciprocity. For example, these light beams are substantially the same wavelength and are directed over the same optical axis. This provides an improved signal-to-noise ratio (SNR) and heightened sensitivity. The SNR is sufficiently high so that a small receiving aperture may be used, in contrast to known direct detection systems. A small receiver aperture may be envisioned as a very small lens capable of being inserted into limited access areas. Since a small receiver aperture can still provide detailed information about the target, the optical components of a coherent system may be made very small and provide related increases in scanning speed and accuracy. For example a coherent optical system using a one-half inch aperture can obtain more information about a target than a four inch aperture used in a direct optical detection system.
Key technologies of Al Ga As laser diodes and fiber optical components are currently enjoying a burst of development for applications in telecommunications. Because of these efforts, recent improvements in the quality of injection laser diodes provide the coherence length and wavelength turning range needed for precision, coherent optical measurement system. The small size of the injection laser diode and high-technology integrated optical assemblies make possible the development of a new family of small, low cost, precise distance measuring devices which are orders of magnitudes more accurate and more reliable than their more conventional counterparts.
High precision, non-contact measurement devices, having a resolution on the order of 25 to 250 nm, are needed to inspect high precision machined components. In contrast, measurement of large objects (e.g., automobiles, airplanes, etc.) may be carried out using coordinate measurement machines (CMMs) and laser trackers. Such precision devices have a resolution in the 1 to 10 micron range.
Laser radar devices for precision measurement applications within this range have been described by Goodwin, U.S. Pat. No. 4,830,486, and Slotwinski and Kenyon, U.S. Pat. No. 4,824,251. Goodwin, for example, discloses frequency modulating a laser, splitting the beam into reference and target components, recombining the beams to create a beat signal (heterodyning) and determining properties of the beat wave by analyzing a pattern of fringes obtained on a detector. Both patents describe fiber optic embodiments of the method.
The principle of operation of an FM heterodyne interferometer for making high precision distance measurements is described in Chien, et al., “Distance and velocity-detection interferometer by using a frequency-modulated triangular-modulated laser diode,” Applied Optics, 1 Jun. 1995, vol. 34, no. 16, (2853–2855), and Imai et al., “Optical-heterodyne displacement measurement using a frequency ramped laser diode,” Optics Communications, 15 Aug. 1990, vol. 18, no. 2, (113–117). These and similar instruments work well for measuring the displacement of ideal reflecting surfaces such as mirrors. The devices which have been demonstrated that are made in accord with the teachings of Chien et al., and Imai et al., and are slow, taking several seconds to integrate data to obtain a usable fringe image. This overly lengthy time for analysis results in these devices being very sensitive to alignment and vibration. When applied to the measurement of position of non-ideal surfaces such as anodized aluminum or the flanks of a tread, which are typically at a 30 degree angle of incidence to the interferometer beam, the signal-to-noise ratio becomes too small to make a reliable high resolution measurement.
As indicated above, the prior art shows that a number of frequency-modulated heterodyne interferometric systems have been experimentally developed for high precision measurement.
The existing art in precision FM laser radar incorporates a single chirp laser source and a polarization maintaining fiber optic geometry with separate local oscillator (LO) and signal paths. The present invention incorporates two major improvements over the art. First, a counter-chirp configuration provides for a much greater insensitivity to vibration induced range errors by providing for a more accurate Doppler correction. Second, by combining the LO and signal paths for two lasers into a single fiber, the fiber optic circuit is both less complicated and less expensive due to fewer components and completely immune to error caused by changes in the LO and signal path lengths due to environmental factors such as temperature changes. For example, it is envisioned that the need for this technology resides in the manufacturing industry (e.g., factories) in which both background vibrations and changing environmental conditions exist. This combination of LO and signal paths provides the additional benefit that the sensor head portion of the unit can be placed in areas of restricted volume since it can be remoted arbitrarily far from the rest of the unit.
Thus, what is needed is a practical optical precision measurement system capable of great accuracy, rapid measurement time, access to tight spaces, flexibility, and reliability. The present invention discloses such a system.